Conjugates of cell binding agents, such as antibodies, with effector groups, such as small cytotoxic agents or cytotoxic proteins, are of immense interest for the development of anti-cancer therapeutics (Richart, A. D., and Tolcher, A. W., 2007, Nature Clinical Practice, 4, 245-25). These conjugates are tumor-specific due to the high specificity of the selected antibodies toward antigens expressed on the cell surface of tumor cells. Upon specific binding to the tumor cell, the antibody-cytotoxic agent conjugate is internalized and degraded inside the target cancer cell thereby releasing the active cytotoxic agent that inhibits essential cellular functions such as microtubule dynamics or DNA replication resulting in the killing of the cancer cell. Various linkers have been employed to link the antibodies with cytotoxic agents with the goal of enhancing the delivery of the agent inside the cell upon internalization and processing of the conjugate, while maintaining the desired stability of the conjugate in plasma. These linkers include disulfide linkers designed with different degrees of steric hindrance to influence their reduction kinetics with intracellular thiol, cleavable peptide linkers such as valine-citrulline linkage, and non-cleavable linkers such as thioether linkage (Widdison, W., et al., J. Med. Chem., 2006, 49, 4392-4408; Erickson, H., et al., Cancer Res., 2006, 66, 4426-4433).
Conjugates of cell binding agents such as antibodies with labels or reporter groups are useful for tumor-imaging applications in cancer patients, immunoassay applications for diagnosis of various diseases, cancer therapy using radioactive nuclide-ligand conjugates, and affinity chromatography applications for purification of bioactive agents such as proteins, peptides, and oligonucleides. The labels or reporter groups that are conjugated with cell-binding agents include fluorophores, and affinity labels such as biotin.
The conventional method of conjugation of the cell-binding agent such as an antibody (Ab) with an effector group (e.g., a cytotoxic agent) or a reporter group (e.g., a radiolabel) linked via a non-reducible linkage (such as thioether linkage) employs two distinct reaction steps with the antibody and necessitates the use of purification steps. In the first reaction step, the antibody is reacted with a heterobifunctional linker bearing two different reactive groups (e.g., X and Y). For example, in one approach, the reaction of an antibody's reactive residues (such as lysine amino residues) with the X reactive group (such as N-hydroxysuccinimide ester) of the heterobifunctional reagent results in the incorporation of the linker with Y reactive group at one or more reactive residues in the antibody (such as lysine amino residues). The initially modified antibody product must be purified from the excess linker or hydrolyzed linker reagent before the next step can occur. In the second reaction step, the linker-modified antibody containing the Y reactive group (such as maleimide or haloacetamide) is reacted with the effector such as an effector group (C) (e.g., a cytotoxic agent) containing a reactive group such as thiol to generate the antibody-effector conjugate, which is again purified in an additional purification step (see, e.g., U.S. Pat. Nos. 5,208,020, 5,416,064, or 5,024,834). Thus, in the above process, at least two purification steps are needed.
Another approach that involves two reaction and purification steps to conjugate antibody with an effector or reporter group uses the reaction of thiol residues in antibody (generated via modification of antibody with thiol-generating reagents such as 2-iminothiolane, or via mutagenesis to incorporate non-native cysteine residues, or via reduction of native disulfide bonds) with a homobifunctional linker Y-L-Y containing Y reactive groups (such as maleimide or haloacetamide).
Major drawbacks of incorporating a reactive group Y such as maleimide (or haloacetamide) in an antibody or peptide are the propensity of the reactive maleimide (or haloacetamide) groups to undergo intra- or inter-molecular reaction with the native histidine, lysine, tyrosine, or cysteine residues in antibody or peptide (Papini, A. et al., Int. J. Pept. Protein Res., 1992, 39, 348-355; Ueda, T. et al., Biochemistry, 1985, 24, 6316-6322), and aqueous inactivation of the Y maleimide group. The undesired intra-molecular or inter-molecular reaction of maleimide (or haloacetamide) groups Y incorporated in antibody with the native histidine, lysine, or cysteine residues in antibody, and aqueous inactivation of the Y maleimide group before the second reaction with the effector or reporter group C give rise to cross-linked proteins or heterogeneous conjugates and lower the efficiency of the second reaction with the effector or reporter group C. The heterogeneous conjugate product—cross-linked protein or peptide generated from the undesired reaction of the initially incorporated group Y (such as maleimide group) with native groups in the antibody or peptides (such as histidine, lysine, tyrosine, or cysteine), or with inactive maleimide residues generated by aqueous inactivation—may have inferior activity and stability than the desired homogeneous conjugate product.
Processes for conjugating antibodies to thiol-containing cytotoxic agents via disulfide linkages have been described previously (see, e.g., U.S. Pat. Nos. 5,208,020, 5,416,064, 6,441,163, U.S. Patent Publication No. 2007/0048314 A1). These processes involve the initial reaction of antibody with a heterobifunctional reagent, followed by a second reaction with a thiol-containing cytotoxic agent. An alternative process has been described in U.S. Pat. No. 6,441,163 B1 in which the disulfide-linked reactive ester of the cytotoxic agent is first purified and then reacted with the antibody, but which involves an additional reaction and purification step starting from the thiol group-containing cytotoxic agent before the reaction step with the antibody.
A further drawback of the current process to make conjugates of cell binding agents is the need for two purification steps, which lowers the overall yield and also makes the process cumbersome and uneconomical for scale-up.
In view of the foregoing, there is a need in the art to develop improved methods of preparing cell-binding agent-drug conjugate compositions that are of substantially high purity and can be prepared avoiding cumbersome steps and by reducing time and cost to the user. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.